1. Field of the Invention
This invention relates to a device and a process for forming a diamond film, particularly to a device and a process for a gas phase synthesis of a diamond film utilizing an arc discharge.
2. Description of the Prior Art
Proposals have been made for a low pressure gas phase synthesis of a diamond film, and these proposed synthesis processes can be broadly classified into the three categories shown below.
First is the hot filament CVD process, which comprises providing a tungsten filament immediately above a substrate heated to 800 to 1000.degree. C., heating the filament to 2000.degree. C. or higher, and blowing hydrogen and a hydrocarbon gas (e.g., CH.sub.4) through the filament against the substrate to thereby form a diamond film on the substrate. Second is the microwave plasma CVD process, which comprises generating a plasma in a gas mixture of hydrogen and hydrocarbon gas by using a microwave of several hundred watts, and growing diamond on a substrate located within the plasma. The substrate is heated by the microwave to have a temperature of 700.degree. to 900.degree. C.
In the above synthesis processes of the hydrogen atoms play an important role, in that hydrogen atoms promote a decomposition of CH.sub.4, and further, effect a selective etching of synthetic components other than diamond, such as amorphous carbon. Third is the synthesis process using an ion beam, by which a diamond film is grown impinging an ion beam of carbon against a substrate.
The diamond film forming processes proposed of the prior art as mentioned above have the following problems Namely, the hot filament CVD process is not satisfactory because of the many occurrences of wire breakages caused by bringing the filament to a high temperature. Also, since the melting point of tungsten is up to about 2000.degree. C., wire breakages will occur when the temperature of the filament is made higher than that, and therefore, a sufficient gas decomposition can not be obtained. In the synthesis process using a microwave plasma, it is difficult to apply same to a sample having a large area because of a limited dimension of the plasma chamber, and because the starting gas, particularly hydrogen, is not sufficiently decomposed. Also, the synthesis process using an ion beam has a problem in that a formed diamond contains many impurities such as amorphous carbon, etc.
Prior Work
The present inventors have proposed a synthesis process for forming a diamond on the substrate by generating an arc discharge between opposed electrodes, permitting the starting gases to pass through the arc discharge to form a gas plasma, forming the gas plasma into a plasma jet gas through a throttle, and blowing the plasma jet gas against a substrate, to thereby deposit a diamond on the substrate, see U.S. application Ser. No. 142813, now U.S. Pat. No. 4,851,254.
In this synthesis process, however, although the synthesis speed is remarkably improved, to improve the purity of the diamond film, the concentration of the introduced hydrocarbon relative to the hydrogen must be lowered, and thus the synthesis speed is lowered.
Briefly, as shown by the data in FIG. 8, a problem exists in that, although by lowering the hydrocarbon concentration the thermal diffusion rate (purity) of the diamond film synthesized is improved, nevertheless the synthesis speed is lowered.